A hydromechanical transmission uses parallel hydrostatic transmission and mechanical power paths in order to reduce the amount of hydrostatic power transmitted. This typically results in a narrow output speed range which is extended by shifting or xe2x80x9cchanging modesxe2x80x9d. Multi-mode hydro mechanical transmissions are typically shifted with the clutch elements at a nominal xe2x80x9csynchronousxe2x80x9d speed condition in order to maintain a continuous input/output speed and torque ratio through the shift. In practice, a smooth shift may not result as the power in the hydrostatic transmission needs to reverse direction during the shift. When reversing the HST power and also maintaining the same nominal synchronous speed of the HST units through the shift, the power direction reversal is accomplished by reversing the direction of hydrostatic pressure in the closed loop. This requires the leakage losses to reverse which must be accompanied by a change in at least one hydrostatic unit""s speed or displacement. Decreasing the time required for this required change in unit speed or displacement has a major impact on improving the quality, or smoothness of the shift.
As disclosed in U.S. Pat. No. 4,341,131 the fastest power reversals are accomplished by making a slightly pre-synchronousxe2x80x9d power-shift and using the engine and vehicle inertia to increase the speed of the fixed hydraulic unit during the shift. The most desirable shift is one in which the variable unit angle is substantially equal before and after the shift, after accounting for pressure and speed differences needed to reverse the HST power flow. This is the quickest and smoothest shift, as far greater power is available to change F-unit speed than V-unit displacement. Pre-synchronous power-shifts typically are done with friction clutches as they are designed to be applied in the presence of a speed differential. However, friction clutches are more expensive than mechanical clutches, and they are more difficult to package effectively. Also, they are not as versatile when accommodating variations in transmission configuration. Mechanical clutches are typically not suitable for making power-shifts with a differential speed, as they cannot slip and the power flow would change too abruptly.
It is therefore a principal object of this invention to provide a method to make smooth shifts in an HMT with mechanical clutches by reducing the speed differential, and momentarily reducing power flow during the shift and then reapplying it before the shift ends.
A further object of the invention is to provide a smooth shift during mode changes of an HMT when using mechanical clutches.
These and other objects will be apparent to those skilled in the art.
A method and apparatus for providing a smooth shift during mode changes of an HMT when using mechanical clutches involves reducing the power into the transmission momentarily when a shift is initiated by cutting fuel to the engine, and also reducing power in the hydrostatic transmission by crossporting the fluid circuit, and then reapplying the power to both the engine and transmission after the clutch has been shifted. The shift sequence also facilitates the initial clutch element speed differential to be reduced by about half as the control allows hydrostatic leakage reduction to increase F-unit speed. During the shift, the F-unit changes speed in an amount required to reverse the direction of power flow and losses in the HST without substantially changing the V-unit displacement.